Sunlight is an abundant component of the natural environment. Sunlight may photo-degrade environmental pollutants to less toxic chemicals, or may generate more reactive and toxic species. Often the light energy determines what kind of species is produced. UV light can break chemical bonds forming free radicals. We have studied the production of NO? and ?and OH radicals from nitrite. These radicals may subsequently react with many organic or inorganic substrates. The UV light also generates free radicals in aquatic humic matter. We have studied the humics from different sources such as rivers, ocean, and soil-based samples, finding that their photochemical properties may differ quite considerably. Visible light is less energetic but may nevertheless start oxidation via photosensitization process. This requires an electron and/or energy transfer energy transfer from a photosensitizer to an appropriate substrate. If oxygen is such a substrate, superoxide or singlet oxygen is generated. Both are very reactive transient and may oxidize many natural and pollutant chemicals. Recently we have studied the photochemical properties of water samples from selected ponds in Minnesota in which frogs with unexplained malformations have been found. It has been suggested that such malformations may somehow be related to the increase in UVB irradiation reaching the earth's surface as a result of the decrease in stratospheric ozone. We have found that a water sample from an affected pond generated larger amounts of singlet oxygen upon UV irradiation than water from a pond where no frog anomalies had been observed. This observation may provide an explanation for the malformations as singlet oxygen is known to damage DNA and is mutagenic. Water samples from other affected are currently being examined.